Field of the Invention:
The present invention relates to an actuating drive for a regulation valve, in particular for a steam turbine regulation valve, as per the preamble of the main claim and to a method for operating the same.
In steam power plants, the steam generated by means of a steam generator is supplied via at least one, generally two to four, steam turbine regulation valves to the steam turbine, in which said steam is expanded, generating mechanical work, and said steam is subsequently supplied to a condenser for condensation purposes. The steam turbine regulation valves are positioned in parallel steam feed lines of the steam turbine in order that the extremely large steam volume flows can be distributed to the different steam turbine regulation valves. Despite the distribution, the steam volume flows per valve remain so large that the valves generally weigh several tons, and correspondingly, the valve body has a high weight, which must be moved by a correspondingly powerful actuating drive.
For the drive of the regulation valves, use is thus generally predominantly made of hydraulic or possibly also pneumatic actuating drives, which have a working cylinder, with a piston rod connected to a piston, as actuator for the valve body of the respective regulation valve, in order that, by retraction of the piston rod into and deployment of the piston rod out of the working cylinder, the valve body is actuated so as, for example in the steam power plant mentioned, to close the flow cross section of the steam turbine regulation valve to a greater or lesser extent.
For the displacement of the piston with the piston rod in the working cylinder, the piston delimits at least one pressure chamber into which a pressurized working medium, either a hydraulic working medium, for example oil, or a pneumatic working medium, for example air, can be introduced in order to displace the piston counter to the force of a spring. The pressure of the working medium is built up by means of a working medium pump in the external working medium circuit, to which the working cylinder is connected by means of at least one corresponding pressure port of the pressure chamber. Since, as is the case in an embodiment of the present invention, the external working medium circuit is generally designed as a closed circuit, said working medium circuit generally has not only a pressure line, via which the pressurized working medium is introduced from the working medium pump into the first pressure chamber, but also a tank line, which tank line conducts working medium out of a second pressure chamber via a second pressure port of the second pressure chamber, which second pressure chamber is positioned on that side of the piston which is averted from the first pressure chamber and which second pressure chamber is delimited with respect to the first pressure chamber by means of the piston, and which tank line feeds said working medium for example to the suction side of the working medium pump. In this way, a double-acting working cylinder for the retraction and deployment of the piston can be realized.
EP 0 055 351 A1 describes a corresponding electrohydraulic actuating drive, having a double-acting working cylinder connected to an external working medium circuit, wherein, in the external working medium circuit, a working medium pump conveys working medium from a working medium reservoir via a check valve into either a first pressure chamber or a second pressure chamber of the working cylinder in order to deploy the piston rod thereof with action in the same direction as the force of a compression spring or in order to retract the piston rod thereof counter to the force of the compression spring. The degree of opening of a turbine regulation valve is determined by means of the position of the piston or of the piston rod. For the rough positioning of the piston in the working cylinder, a binary throughflow switch in the form of a four-way switching slide valve with three switching positions is provided, the two control lines of which are connected to the pressure chambers on both sides of the piston in the working cylinder, and which is furthermore connected via a pressure line to the pressure side of the pump and by means of a tank line to the working medium reservoir. For the fine positioning of the piston, an electrohydraulic transducer in the form of a pilot-controlled servo valve is provided.
A disadvantage of the actuating drive as per EP 0 055 351 A1 is that the efficiency is limited by the throttling losses at the control edges of the throttling directional valves, which are in the form of continuously adjustable valves. Furthermore, high demands are placed on a constant supply pressure in order that the position of the piston in the working cylinder can be regulated in an exact manner.
DE 40 30 107 A1 describes a corresponding actuating drive in which throttling losses owing to continuously adjustable valves are substantially avoided. In said actuating drive, which likewise has a working cylinder with two pressure chambers which are connected to an external working medium circuit with a working medium pump, the working medium pump, which is designed as a constant-displacement pump and which is driven by means of a rotational-speed-regulated motor, is used to pump working medium via a check valve, which is open in the conveying direction, into a first pressure chamber of the working cylinder, such that the piston with the piston rod retracts counter to the force of a compression spring. A disadvantage is that the deployment of the piston rod is effected exclusively by the force of the compression spring, whereby the dynamic actuation characteristic is asymmetrical and, in the direction of the spring force, is defined only by the hydraulic-mechanical configuration of the flow resistance and cannot be influenced by electrical control signals. Furthermore, the working medium pump must be capable of starting up counter to the pressure prevailing in the pressure chamber of the working cylinder over all rotational speed ranges.
It is also the case in the actuating drive as per EP 2 620 655 A1 that throttling losses in continuously adjustable valves are substantially avoided, and furthermore, a working medium pump driven by a servomotor is provided in the external working medium circuit, which working medium pump is of variable rotational speed and can pump working medium from a first pressure chamber of the working cylinder into a second pressure chamber of the working cylinder and vice versa in order to thereby rapidly adjust the desired position of the piston and thus of the piston rod. Here, however, there is likewise the disadvantage that the working medium pump must be capable of operating counter to the working medium pressure in the two pressure chambers of the working cylinder over all rotational speed ranges, and a relatively expensive synchronous servomotor is required for dynamic and precise control. Furthermore, the working medium pump must be capable, in the static state of the piston, of imparting a holding pressure counter to the spring which exerts pressure on the piston. The torque required for the holding pressure at low rotational speed is a thermally unfavorable operating situation for the motor of the working medium pump. Also, the working medium pump tends to overheat in the case of high pressures and low volume flows, such as are required for the fine positioning of the piston.